1. Field of the Invention
The present invention disclosed in the specification relates to a thin film transistor and a method of making thereof. Further, the present invention relates to a method of making a display device of an active matrix type utilizing thin film transistors.
2. Description of Related Art
A thin film transistor has been known as a device for constituting an active matrix type liquid crystal display device. Particular attention is being paid to technologies utilizing a thin film transistor using a silicon thin film having crystallinity.
A thin film transistor using a silicon film having crystallinity is characterized in that a high speed operation is feasible and a CMOS (Complementary Metal Oxide Semiconductor) circuit can be constituted.
When a thin film transistor using a silicon thin film having crystallinity is used, an active matrix circuit and peripheral drive circuits for driving the active matrix circuit can be integrated on one sheet of a glass substrate (or quartz substrate) by making use of such a characteristic.
However, a crystalline silicon thin film provided in the current technology is not in a single crystal state but in a polycrystal state or a microcrystal state. Defects or impurities are included in such a film (referred to as crystalline silicon film) at a comparatively high level. Accordingly, there poses a problem where the structure of a bonded portion of different conductive materials is electrically inferior. The problem gives rise to a factor causing an aging change (generally amountable to deterioration) of an OFF current or properties in the operation of a thin film transistor.
For example, the problem where the OFF current (a current made to flow between a source and a drain in the OFF operation) is comparatively large is a problem common to thin film transistors of a P-channel and an N-channel type. Furthermore, the mobility of carriers in the P-channel type thin film transistor is lower than that of carriers of the N-channel type one. Also, there poses a problem in the N-channel type transistor where deterioration by hot carriers (deterioration thereof particularly at a bonded portion) is significant.
It is preferable to attain simultaneous resolution of the above-described problems when an integrated circuit using thin film transistors are constituted. Especially, when P-channel type and N-channel type transistors are simultaneously formed (separately formed) on a same substrate, a difference in characteristics between the P-channel type transistors and the N-channel type ones needs to correct.
The reason is that a circuit, (generally constituted based on a CMOS circuit) having excellent characteristics cannot be obtained when only the N-channel transistor or the P-channel transistor is provided with a low OFF current characteristic or is provided with a high mobility.
It is a problem of the present invention disclosed in the specification to provide a thin film transistor having a low OFF current value. Also, it is a problem thereof to provide thin film transistors of a P-channel type and an N-channel type where the difference in characteristics is corrected.
According to one aspect of the present invention disclosed in the specification, as illustrated in a specific constitution example of FIG. 4, there is provided a semiconductor device which is a P-channel type thin film transistor having a channel forming region 140 and a drain region 150, wherein an impurity region 149 having a stronger p-type characteristics than the drain region is arranged between the channel forming region and the drain region.
The low OFF current characteristic can be obtained by arranging the impurity region 149 having the stronger P-type characteristics. The P-type behavior more potential than the P-type behavior of the drain region 150 signifies that the impurity region 149 is provided with the property as a P-type semiconductor more potential than that of the drain region 150. The intensity of the property as the P-type semiconductor can be compared by a hole density (density of majority carriers) or conductance. That is, the P-type semiconductor having a high hole density and a high conductance has the stronger property as a P-type semiconductor. In FIG. 4, the relative degree of the intensity of the property as the P-type semiconductor is designated by notations P+ or P++.
In the above-described constitution, an impurity providing an N-type behavior is included in the region 149 and the drain region 150. This is because an impurity providing the N-type behavior is simultaneously implanted in forming an N-channel type thin film transistor as illustrated by FIGS. 3(A) and 3(B).
As is apparent in reference to FIGS. 3(A) and 3(B), the concentration of the impurity providing the N-type behavior that is included in the region 149 having the potential P-type behavior is smaller than the concentration of the impurity providing the N-type behavior that is included in the drain region 150.
Also, as illustrated in FIG. 4, the impurity ions for providing the P-type behavior are simultaneously implanted to the region 149 and the region 150 and therefore, the region 149 having the potential P-type behavior and the drain region 150 include the impurity providing the P-type behavior by concentrations substantially the same as each other.
According to another aspect of the present invention, as illustrated by a specific constitution example of FIG. 4, there is provided a semiconductor device which is a P-channel type thin film transistor having a channel forming region 140 and a drain region 150, wherein the drain region 150 includes an impurity providing an N-type behavior, a region 149 including the impurity providing the N-type behavior by a concentration lower than that of the drain region 150 is arranged between the channel forming region 140 and the drain region 150, and the region 149 including the impurity providing the N-type behavior by the concentration lower than that of the drain region 150, is provided with the P-type behavior more potential than that of the drain region 150.
According to another aspect of the present invention, as illustrated by a specific constitution example of FIG. 4, there is provided a semiconductor device where a P-channel type and an N-channel type thin film transistor are formed on a same substrate, an impurity region 145 having a P-type behavior more potential than that of the drain region is arranged between a channel forming region 134 and a drain region 146 in the P-channel type thin film; transistor, and a low concentration impurity region 136 including the impurity providing the N-type behavior by a concentration lower than that of a drain region 127 is arranged between a channel forming region 137 and the drain region 127 in the N-channel type thin film transistor.
Although an example using a glass substrate is shown in FIG. 4, the above-described constitution can be utilized to a substrate having other insulating surface and an integrated circuit having a multilayered structure.
In the above structure, the impurity region 145 having the potential P-type behavior include the impurity providing the N-type behavior by a concentration substantially the same as that of the low concentration impurity region 136, a source and a drain region 143 and 146 of the P-channel type thin film transistor, include the impurity providing the N-type behavior by a concentration substantially the same as those of a source and a drain region 129 and 127 of the N-channel type thin film transistor, and the impurity region 145 having the potential P-type behavior and the source and the drain regions 143 and 146 of the P-channel type thin film include the impurity providing the P-type behavior by substantially the same concentrations.
In the above-described constitution, the low concentration impurity region designated by numeral 136 of the N-channel type thin film transistor is a region generally referred to as an LDD (Light Dope Drain) region. According to another aspect of the present invention, as illustrated by a specific constitution example of FIG. 4, there is provided a semiconductor device where a P-channel type thin film transistor and an N-channel type thin film transistor are formed on a same substrate, a region 145 including the impurity providing the P-type behavior by a concentration substantially the same as that of a drain region 146 and having the P-type behavior more potential than that of the drain region 146 is formed between a channel forming region 134 and the drain region 146 in the P-channel type thin film transistor, and a low concentration impurity region 136 including the impurity providing the N-type behavior by a concentration lower than that of a drain region 127 is arranged between a channel forming region 137 and the drain region 127 in the N-channel type thin film transistor.
According to another aspect of the present invention, as illustrated by a specific constitution example of FIG. 4, there is provided a semiconductor device having the constitution where an active matrix circuit (in which a P-channel type thin film transistor on the right is arranged) and a peripheral drive circuit (in which a P-channel and an N-channel type thin film transistor on the left are arranged) for driving the active matrix circuit, are integrally arranged, where a P-channel type thin film transistor is arranged in the active matrix circuit, a P-channel type and an N-channel type thin film transistor are arranged at the peripheral drive circuit, impurity regions having a P-type behavior more potential than that of drain regions are arranged between channel forming regions and the drain regions of the P-channel type thin film transistors, and a low concentration impurity region 136 including the impurity providing the N-type behavior by a concentration lower than that of a drain region 127 is arranged between a channel forming region 137 and the drain region 127.
In the above-described constitution, the channel forming regions of the P-channel type thin film transistors are designated by numerals 134 and 140. The drain regions of the P-channel type thin film transistor are designated by numerals 146 and 150. Further, the impurity regions having the P-type behavior more potential than those of the drain regions of the P-channel type thin film transistor are designated by numerals 145 and 149.
According to another aspect of the present invention, as illustrated by a specific constitution example of FIG. 7, there is provided a semiconductor device where a P-channel type thin film transistor and N-channel type thin film transistors are formed on a same substrate, an impurity region having the P-type behavior more potential than a drain region is arranged between a channel forming region and the drain region in the P-channel type thin film transistor, and offset gate regions are arranged between channel forming regions and drain regions in the N-channel type thin film transistors.
According to another aspect of the present invention, as illustrated by fabrication steps of FIG. 1(A) through FIG. 5(B), there is provided a method of making a semiconductor including the steps of forming a thin film semiconductor on a substrate (FIG. 1(A)), forming active layers 104 through 106 of P-channel type thin film transistors and an N-channel type thin film transistor by using the thin film semiconductor (FIG. 1(B)), forming N-type regions 124, 126, 127, 129, 130 and 132 and low concentration impurity regions 133, 135, 136, 138, 139 and 141 added with the impurity providing the N-type behavior by a concentration lower than that of the N-type regions in a total of the active layers (FIGS. 3(A) and 3(B)) and adding the impurity providing the P-type behavior to the N-type regions and the low concentration impurity regions in the active layers constituting the P-channel type thin film transistors (FIG. 4).
According to another aspect of the present invention, as illustrated by fabrication steps of FIG. 1(A) through FIG. 5(B), there is provided a method of making a semiconductor including the steps of forming a plurality of regions 124, 126, 127, 129, 130 and 132 having the N-type behavior by adding the impurity providing the N-type behavior to semiconductor thin films (FIG. 3(A)), forming a plurality of low concentration impurity regions 133, 135, 136, 138, 139 and 141 including the impurity providing the N-type behavior by a concentration lower than that of the regions having the N-type behavior by adding the impurity providing the N-type behavior by a concentration lower than that in the preceding step (FIG. 3(B)), and inverting at least portions of the plurality of regions having the N-type behavior into regions having the P-type behavior and simultaneously inverting at least portions of the plurality of low concentration impurity regions into regions having the P-type behavior more potential than that of the regions inverted to have the P-type behavior by selectively adding the impurity providing the P-type behavior.
According to another aspect of the present invention, as illustrated by fabrication steps of FIG. 1(A) through FIG. 5(B), there is provided a method of making a P-channel type thin film transistor comprising the step of adding the impurity providing the P-type behavior to a semiconductor film having regions 130 and 132 having an N-type behavior and low concentration impurity regions 139 and 141 including the impurity providing the N-type behavior by a concentration lower than that of the regions 130 and 132 (refer to FIGS. 3(A) and 3(B)) whereby the regions 130 and, 132 having the N-type behavior are inverted to provide the P-type behavior to thereby form source and drain regions 147 and 150 and whereby the low concentration impurity regions 139 and 141 are simultaneously inverted to regions 148 and 149 having the P-type behavior more potential than that of the source and drain regions (FIG. 4) by which the region 149 having the potential P-type behavior is arranged between the source region 150 and a channel forming region 140.
According to the present invention disclosed in the specification, the shape of the active layers is not limited to the one that is patterned linearly. For example, the shape of the active layers may be that of a channel type one, an angle type one, or ones having complicated shapes.
Further, although a planer type thin film transistor is mainly shown in the specification, the present invention disclosed in the specification may be utilized also in the stagger type or the inverse stagger type.
The present invention disclosed in the specification is applicable not only to an active matrix type liquid crystal display device but to other active matrix type flat panel displays.
Additionally, the present invention disclosed in the specification can be utilized to the constitution of thin film transistors arranged in an integrated circuit having a multilayers structure, or a circuit using thin film transistors arranged in an integrated circuit having a multilayered structure.
As illustrated by FIG. 4, according to the P-channel type thin film transistor, the region 144 having the P-type behavior more potential than that of the drain region 146 is arranged between the channel forming region 134 and the drain region 146. In this way the P-channel type thin film transistor having the low OFF characteristic can be provided.
Also, according to the N-channel type thin film transistor, the low concentration impurity region 138 is arranged between the channel forming region 137 and the drain region 127. In this way, the N-channel type thin film transistor having the low OFF characteristic and where deterioration is restrained can be provided.